Electrophotographic image forming devices form latent images on an image carrier by scanning the image carrier using laser light deflected by a rotating polygonal mirror. This rotating polygonal mirror is driven by a motor that rotates constantly in one direction. Conventionally, three Hall elements have been required to detect the rotation direction of the motor. Decreasing the number of the Hall elements to one can reduce the manufacturing cost, but increases the amount of time necessary for detecting a reverse rotation.
In view of the above, Japanese Patent No. 3993502 proposes the invention whereby, when a small current flows through any two phases of three phases, a polarity of voltage induced in the remaining one phase is detected. When the detection results of the two phases out of the detection results of the three phases match the expected results, a rotor is rotated in disregard of the detection result of the remaining one phase.
However, the invention of Japanese Patent No. 3993502 is problematic not only in that a detection circuit that detects voltage induced in the three phases is costly, but also in that the detection takes time.